Interdigital magnetron



Dec. 9, 1947. BURNS 2,432,466

INTERDIGITAL MAGNETRON F iled Nov. 29, 1946 j 1 G i U- J U H U U U U INVENTOR.

Patented Dec. 9, 1947 UNITED STATES V OFFICE 2,432,486- 7 INTERDIGITAL MAGNETRON Laurence Burns, Swampscott, Masst, assignor to Sylvania Electric Products Inc., Salem, Mass, a corporation of'Massachusetts Application November 29, 1916,. Serial No. 713,012

Claims. I

This invention relates to microwave devices and particularly to microwave generators, such as interdigital magnetrons and the like.

Such devices have previously comprised an anode having two oppositely facing sets of conducting fingers or teeth arranged "on the surface of a cylinder or the like, the sets being interrneshed but not in contact, with the circuit hetween the two sets'of teeth completed through a resonant cavity.

In such an arrangement there are radial: modes of oscillation and circumferential or angular modes. In the latter, the field strength varies around the cylinder formed by the teeth, and the coupling is best when the coupling loop is placed in the plane of the voltage maximum. However, as the coupling is increased, or the anode voltage raised, the voltage ma-Xima tend to shift away from the loop, and may eventually shift 90, under-which condition the device may oscillate desired, as in the copending application of Henry strongly with no power output to the coupling My invention, is in preventing this by opening the circuit around the ring at the current nodes, preferably by making a slot or opening in the ring at the nodes, the slot extending far enough into this cavity to insure that the opening/in the ring is not short-circuited by the cavity itself. The slot or opening in the ring should be great enough to prevent s'hort-circuiting itself by the capacity between the open ends of the ring, and the opening preferably extends into the cavity sufficiently to insure that a wave going around the circumference of the opening will be delayed just a little more than a wave-length so that the current on one side of the opening in the ring will have a small value in one direction and on the other side a small value in the other direction, to preserve the desired current distribution around the ring. This allows oscillation only for those modes having current nodes at the slotted positions.

Further objects advantages and featuresofthe invention will be apparent from the following specification, taken in conjunction with the accompanyin drawing. I

Figure 1 is a developed view of the split ring of my invention, with the voltage and current distributions plotted along it.

Figure 2 is a view, partly in section, of the toothed rings with discs attached thereto for connection to the resonant cavity.

Figtue 3 is a sectional view "in which the split rings are connected to oppositely placed waveguides as resonators; and

Figure 4 is a sectional View of the waveguide along the plane of line l l of Figure 3.

In Figure 1, the current, curve I is seen topass through its node at the gap G in between sections l and 2 of the ring l--2, or sections 3 and J. McCarthy, filed in the United States Patent (mice: on or about October 30, 1946, for an Interdig'ital magnetron. The interdigital magnetron itself is well described in the McCarthy application, which is incorporated herein by reference, and in my application I will describe herein only my improvements thereon. A magnetron according to my invention may have a cathode concentric with the ring, an enclosing envelope also concentric therewith, and the usual magnet for producing a field between the anode and the cathode, as shown in said McCarthy application. The enclosing glass tube !5 is herein shown broken, for convenience.

In Figure 3, the concentric resonator discs I, 8 are replaced by the top and bottom (e. g., It,

' 19) of waveguides Hi, I! each of which is connected at its top and bottom to one of the halves 1, 2,3, 4 of the split rings l-Z and 3-4. That is half-ring I and 3 go to one waveguide l6, halfring 2 and 4 to the other waveguide ll. There is then no direct connection between the waveguides It and I1 inside the tube, but they may be coupled together outside the tube. The glass tube 15 encloses the internal elements of the tube, the wave-guide extending through it. In some cases, one may find it possible to connect the two wave-guides together in the old splitanode magnetron circuit; for lower frequencies, thus superposing a lower frequency modulation on the high frequency wave.

In general, the disc type tube of Figure 2 will be easier to manufacture than the wave-guide type of Figure 3, because of the simpler sealing operations.

The'circumference of each of the openings 9, l 0, H, I2 is preferably sufiicient to cause a slight phase difference between the currents on opposite sides of the gaps -6 in the rings I--2 and 3-4, so that the currents on opposite sides of the gaps 6' will be small, equal and opposite. This can be accomplished by making the aps long enough to insure a little more than a wavelength delay to any wave going around or through the gap.

In some cases, it may be desirable to place small baflles over the openings 9, I0, ll, l2 to. prevent radiation losses therethroug-h. -Such reflectors or baliles may be supported from one end to prevent shorting the gap.

In some cases, it may be desirable to make the width of the wave guides I6, I! in Figure 3 greater than the diameter of the tube, or to split the discs I, 8, taking appropriate termination:

precautions to prevent loss of energy- When a tooth is to be shorted as in the McCarthy application, it may at times be,desirpolarities, the shorted tooth bein of substantially no polarity, that is at zero potential. Of course, the potential may vary somewhat along the tooth itself. The other teeth may then be alternately connected to opposite rings, except near any shorted tooth.

If such a shorted tooth be used in addition to the slots ofthe present invention, and if it nonetheless be desired to vary the wavelength of oscillation by moving the ring-supporting discs 1, 8, by a diaphragm arranged as part of the tube envelope or the like in known manner, the shorted teeth will have to be flexible in their contacting arrangement. For example, the shorted tooth may be attached to one ring and its end -may fit into and contact a hollow longitudinally split conducting cylinder attached to the other ring. If the teeth are of rectanguiar cross-section, as shown, the cylinder will also be of rectangular cross-section to fit snugly but slidably around the tooth, bearing against it for contact.

In some cases it may b desirable to have the slots of size other than a value giving a little more than a wavelength delay or phase shift, a slot extending just through the ring, or through the ring and extending just a little into the annular discs attached thereto often being desirable. If the ring is wide enough, the slot may exte'nd'only part way into it in exceptional cases. The slot should be sufficient to insur a break in the current distribution curve around the ring if any mode is used other than one which makes the current node or minimum come at that point. The slot in the ring will not greatly aifect the current distribution for the desired mode-the current is zero or a minimum at that point anyway. But a mode in which the current will not be a minimum'at that point will be hard to establish.

If a coupling loop is used to take power from my magnetron, it will generally be placed in or near a plane through the slots and the longitudinal axis of the toothed cylinder. That is, the loop may be in the plane of the voltage maxima.

Although the slots 9, M, H, L. are shown flared in the drawing they may, of course, have straight sides if desired.

In some cases, an uneven number of teeth may be desirable, in which. case one current node may occur between teeth, and in convenient position for slotting, and the other node may occur at a tooth and hence in less convenient position. In such a case a slot at the node in between teeth will generally b sufficient, although a slot extending vertically down the tooth at the other node for a short distance may be helpful. Current may then flow into the tooth, but not across the ring at the top of the tooth.

Additional slots for harmonics may be used if desired.

What I claim is:

' tions of the rings.

2. An interdigital magnetron construction comprising an anode comprising two rings in register with each other, oppositely placed sets of teeth extending toward each other from said rings, each set coaxial with the other and each set comprising a series of electrically connected spaced teeth mounted longitudinally along the surface of a cylinder, the teeth of the sets, being in general spaced from each other, a pair of spaced metallic'discs extending from and connected to said rings, each ring and its corresponding disc being slotted radially at some of the approximate current nodes of a mode of oscillation.

3. An interdigital magnetron construction comprising an anode comprising two rings in register with each other, oppositely placed sets of teeth'extending toward each other from said rings, each set coaxial with the other and each set comprising a series of electrically connected spaced teeth mounted longitudinally along the surface of a cylinder, the teeth of the sets being in general spaced from each other, a pair of spaced metallic discs extending from and connected to said rings, each ring and its corresponding disc being slotted radially at tWo diametrically opposite points.

4. An' interdigital magnetron comprising an anode comprising two rings in register with each other, oppositely placed sets of teeth extending toward each other from said rings, each set coaxial with the other and each set comprising a series of electrically connected spaced teeth mounted longitudinally along the surface of a cylinder, the teeth of the sets being in general spaced from each other, a pair of spaced metallic discs extending from and connected to said rings, each ring and its corresponding disc being slotted radially at one place.

5. An interdigital magnetron construction comprising an anode comprising two rings in register with each other, oppositely placed sets of teeth extending toward each other from said rings, each set coaxial with the other and each set comprising a series of electrically connected spaced teeth mounted longitudinally along the surface of a cylinder, the teeth of the sets being in general spaced from each other,and a pair of metallic pieces extending from and connected to said rings, each ring and its corresponding metal piece being slotted radially at two diametrically opposite points, the slots exending to the edges of the metallic pieces.

LAURENCE BURNS.

REFERENCES CITED The following references areof record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,414,084 Bowen Jan. 14, 1947 2,424,886 Hansell July 29, 1947 FOREIGN PATENTS Number Country Date 216,807 Switzerland Jan. 5, 1942 construction 

